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Forecasts of Niño 3 Tropical Pacific SST
Using a Low Order Coupled Ocean-Atmosphere Dynamical Model
contributed by Richard Kleeman
Bureau of Meteorology Research Center, Melbourne, Australia
A simple coupled ocean/atmosphere model has recently been developed at the Bureau of
Meteorology Research Center (BMRC) (Kleeman 1993) in order to explore the physical basis of
ENSO predictability. In particular, a variety of very simple ocean models with varying
thermodynamical equations governing SST have been coupled to a simple atmospheric model
which performs well when forced by a full range of ENSO SST anomalies (Kleeman 1991).
The coupled models are somewhat similar to that of Cane and Zebiak (Cane and Zebiak 1987),
but differ in aspects of the coupling, atmospheric convection and heating, and ocean
thermodynamics.
The hindcast skill of these models was tested using the ocean models initialized at regular 3 month
intervals between January 1972 and July 1986 using FSU winds, and it was determined that
optimal skill was obtained when the ocean model SST was determined purely by equatorial
thermocline perturba-tions.
The initialization of the coupled model was improved (Kleeman et al. 1995) by using a space-time
variational (adjoint) technique to assimilate sub-surface thermal data, as well as the usual wind
data, into the ocean model. This resulted in an increase in the model skill. The skill of the
improved system and persistence for forty forecasts with 1982-1991 start dates is shown in Fig. 1
of the March and June issues of this Bulletin.
Displayed in Fig. 1 below is the most recent forecast of Niño 3, which uses the FSU winds up to
October 1997 and sub-surface thermal data up to September 1997 to initialize the model. The
forecast shows that strongly warm conditions are expected to persist through to early 1998.
Subsequent to this the warm event is expected to subside, although somewhat warm conditions
are expected to persist until late 1998 when cold conditions are forecast to develop. Compared to
the previous forecast, mid 1998 is now expected to remain warmer.
Cane, M. and S.E. Zebiak, 1987: Prediction of El Niño events using a physical model.
InAtmospheric and Oceanic Variability, H. Cattle, Ed., Royal Meteorological Society Press,
153-182.
Kleeman, R., 1991: A simple model of the atmospheric response to ENSO sea surface
temperature anomalies. J. Atmos. Sci., 48, 3-18.
Kleeman, R., 1993: On the dependence of hindcast skill on ocean thermodynamics in a coupled
ocean-atmosphere model. J. Climate, 6, 2012-2033.
Kleeman, R., A.M. Moore and N.R. Smith, 1995: Assimilation of sub-surface thermal data into an intermediate tropical coupled ocean-atmosphere model. Mon. Wea. Rev, 123, 3103-3113.
Fig. 1. Current forecast for Niño 3 SST using the BMRC low order coupled model. FSU winds
up to October 1997 and sub-surface thermal data up to September 1997 are used for intialization.
